Associate one emitter with a specific curve
To ensure that particles from an emitter are following a curve guide force field do the following:


Make a simple emittor object e.g. a cube
Change to edit mode, select one vertex and add this single vertex to a vertex group
In the particle systems tab, scroll down to Vertex Groups and select the vertex group with the single emitter vertex - this will cause the particles to be emitted from just this one vertex
Place the emitter vertex close to the curve guide path
(Optional) Adjust the force field settings to control the effective distance of the field (to avoid overlaps with other fields)
If you need multiple emitters and paths you can parent the emitter to the path and duplicate as needed.


Setting this up, can be a bit tricky sometimes, since it can happen, that particles drop out of the force field, but with a bit of fiddling it can be done.



I used this technique to create the arrows in this video: http://rheologic.net/en/urban-wind-assessment#wind-effects